Tweed ODS... micro?

[nickfl]

The 6BM8 is still in production. It's one of my favorite tubes. It has a great bass tone for a low wattage tube. I'd aim for around 310v or less on the plates IF I was attempting this idea. I'd also use a 5751 instead of the 12AX7 in V1. And I'd eliminate some of the components between the DPDT switch and the trim of the OD (compare this schematic to original TOS).

The PPIMV will help in not overdriving the 6BM8 pentodes. It might be worth changing the 1M resistors in the LTPI to 470k to 680k range also?

I am not suggesting you change your build but simply posting this idea if someone happens to be interested? Note there is an editable ExpressSCH schematic attached.

BTW, Doug Hoffman sells layout boards for the TOS

With respect, 10thtx

What are your thoughts on a choke in the power supply? Your schematic doesn't have one, the original Tweed ODS has one between plate and screen nodes, and mine has one before the first power supply node. I'm inclined to either move mine to between the plate and screens (now that I'm going to use pentodes and actually have some screens to work with) or eliminate it entirely. Either way, I'll be able to remove one stage of filtering and save some space in the chassis.

[nickfl]

I'm reworking my schematic for 6bm8 tubes etc right now. Putting a choke inline between the plate node and screen node, and I notice that I have roughly the same voltage for the screens that I need for the v2b triode in the overdrive section of the preamp. Would it be a problem to supply the screens and that triode from the same node? I could always add another filter section and supply them from seperate nodes, but it would be nice to keep the power supply from getting any more complicated.

[nickfl]

Ok, here's an updated schematic:

Tweed overdrive micro Schem rev 2 6bm8.png

I've updated the Power section and PI values for 6BM8 tubes, moved the choke so its between the Plate and screen power nodes, combined the Pi and V2b power nodes to simplify the power supply, and recalculated the power supply dropping resistor values to compensate for all these changes. I've also corrected the OT specs, which were incorrect on the first schematic. Lastly, I added NFB and a presence control. I had though about NFB on my original schematic but didn't include it, but I went ahead and took the values from the Bassman mini and adjusted them for a 16 ohm speaker load (correctly I hope). It is implemented with a presence control now, but I'm not sure I really want that and may want to just make it some sort of adjustable NFB control instead.

[10thTx]

Regarding a choke or not, .......... just a personal preference in tone and I don't necessarily seeing one approach "better" then the other. I'd probably use a choke IF I had room, but if not I'd just use a resistor and enjoy the amp that way.

I think your Edcor trannie with 210-0-210 and 120ma will work fine.

I don't know how to answer you B+ rail and node question. I would not do it the way you suggested and I would stick with the method that I am familiar with that has worked for me on numerous builds that are quiet at idle (similar to the schematic I shared with you). Maybe your approach would work fine, I'm just not familiar with that?

I would not get hung up on trying to duplicate the original TOS preamp and overdrive stage voltages. Getting the plates on the 12A_7's and 5879 in the 140v to 160v range would probably suffice.

I noticed the PPIMV is removed in your schematic. Again, that is a personal preference thing. I personally would want it as 7-9 watts can still get very loud and also because the 6BM8's can be overdriven more easily then a 6V6 (for example). I think your amp may sound fantastic even without the PPIMV.

With respect, 10thtx

[10thTx]

BTW, I found two sound clips of DaGeezer's original Little Wing mini-bassman

http://www.musicwebtown.com/jallenshaw/46763

This may give you some idea of what your "clean" channel could sound like or at least the 6BM8's sound like?

Here is a soundclip of my 56T (HoSo56 with reverb) using 6BM8's. Mediocre playing and about 2 minutes too long but you get the idea of the amount of bass this
small tube has. Maybe start the soundclip around 1:55 into it. Both the slide and regular playing are on the 56T

http://www.soundclick.com/html5/v3/play ... i&newref=1

BTW, your schematic has an error. You have 2.2M grid resistors for the 6BM8 pentodes. IF you are not using the PPIMV, then you'd want 220k grid resistors.

[joeboo88]

This is good reading, thanks for all the info.
I built the mini bassman with 6bm8 tubes and this is my favourite amp, as it has oodles of bass, which is what I love.
I used a 270 0 270 Edcor PT with SS rectification, and I think my plates Are at 350 I think....
I know it's a bit much for these tubes, but it's an experiment and I have been playing them for a month so far. I was told by a good friend these tubes are very rugged, and so far that's true.

I am going to build the Hoso next also with the 6bm8's using 10htx's schematic from EL34 world.
Thanks for that.....Joe

[nickfl]

Regarding a choke or not, .......... just a personal preference in tone and I don't necessarily seeing one approach "better" then the other. I'd probably use a choke IF I had room, but if not I'd just use a resistor and enjoy the amp that way.

I think your Edcor trannie with 210-0-210 and 120ma will work fine.

I don't know how to answer you B+ rail and node question. I would not do it the way you suggested and I would stick with the method that I am familiar with that has worked for me on numerous builds that are quiet at idle (similar to the schematic I shared with you). Maybe your approach would work fine, I'm just not familiar with that?

I would not get hung up on trying to duplicate the original TOS preamp and overdrive stage voltages. Getting the plates on the 12A_7's and 5879 in the 140v to 160v range would probably suffice.

I noticed the PPIMV is removed in your schematic. Again, that is a personal preference thing. I personally would want it as 7-9 watts can still get very loud and also because the 6BM8's can be overdriven more easily then a 6V6 (for example). I think your amp may sound fantastic even without the PPIMV.

With respect, 10thtx

I'm thinking about eliminating the clean channel level control and adding the PPIMV. I'm kind of inclined to do one or the other as it just has too much going on on the front panel otherwise and I think the post PI master is probably a lot more useful than a level control that seems like it would be of limited use anyway.

[nickfl]

Another revision, with the clean level removed, PPIMV implemented, and the screen and OD triode nodes seperated again. I thought about the power supply design some more and it did seem like combining the screen node with the node for that triode was asking for trouble, especially in a high gain design and one more filter cap isn't going to be that hard to fit in the chassis.

Tweed ODS micro Schem rev3 PPIMV.png

[10thTx]

I think you will have a fantastic amp! With respect, 10thtx

[nickfl]

Double post...

[nickfl]

Here is a layout of the schematic in my last post:

Tweed ODS micro layout.png

It is intended to be mounted marshall style, on the bottom of a head cabinet, but with the input/preamp side on the left when you are facing the control panel.

I've gone ahead and ordered a blank chassis, Edcor transformers, and components, so the ball is rolling on this one...

[10thTx]

You've done a great job with the layout! Well done.

I prefer audio over linear for anything that functions like a volume pot. It makes for a smoother and more gradual increase in volume. I especially would want it for the 1st volume pot and the PPIMV.

I would probably mount the 33k input grid resistor right on tube socket and the 8.2k directly on the 6BM8 pentode sockets.
Having said that, my guess is how you have done them is fine.

Looking forwards to seeing more about your build! With respect, 10thtx

[10thTx]

It is intended to be mounted marshall style, on the bottom of a head cabinet, but with the input/preamp side on the left when you are facing the control panel.

IF you discover that your layout needs to be changed in it's direction for some reason, one can use "PAINT" to flip horizontally to do so.

with respect, 10thtx

[nickfl]

Does anyone have experience with the Svetlana, winged C 6BM8/6F3P tubes? I'm shopping around for a set of Power/PI tubes for this build. NOS US or British 6BM8s on ebay are a little pricier than I had hoped for and I have heard mixed reviews on the current production EHX tubes. I keep seeing winged C 6F3P (advertised as 6BM8 or ECL 82) tubes at good prices and I've heard good things about some of their other tubes, so I am wondering if I should give them a try? I tried searching for reviews, but the best I could find was some audiophile arguments about them and not much of that even...

[10thTx]

I've had a pair of 6BM8 that were old stock (don't remember the brand) and two sets of EHX 6BM8 new production 6BM8's. I don't remember any difference in
tone and I enjoyed the tone of the EHX 6BM8 tubes which were trouble free for me.

with respect, 10thtx